amazon: Clarify log message
amazon: Clarify log message

The previous message could be interpreted as "failed to launch
machines".

kklin/kelda,ejj/quilt-dev,LuiseV/quilt,quilt/quilt,kklin/kelda,kayousterhout/quilt,kayousterhout/quilt,kklin/quilt,kklin/quilt,ejj/quilt-dev,NetSys/quilt,tejasmanohar/quilt,kelda/kelda,kklin/quilt,LuiseV/quilt,NetSys/quilt,tejasmanohar/quilt,kklin/kelda,NetSys/quilt,NetSys/quilt,kayousterhout/quilt,ejj/quilt-dev,quilt/quilt,quilt/quilt,kklin/quilt,kelda/kelda,kklin/kelda,NetSys/quilt,LuiseV/quilt,tejasmanohar/quilt,LuiseV/quilt,LuiseV/quilt,kelda/kelda,quilt/quilt,kelda/kelda,tejasmanohar/quilt,kayousterhout/quilt,ejj/quilt-dev